xkb *_Latch key doesn’t work well on Wayland

[ariasuni]

Summary

Ergo-L (fr-ergol) uses a key as ISO_Level5_Latch, which works as both as either a modifier (if pressed and maintained at the same time as another key) or a dead key (if pressed and released before hitting another key).

But with IBus enabled, it doesn’t work as expected, in Wayland applications (works in Electron apps using XWayland).

(same issue than fcitx/fcitx5#1301 but with slightly different behavior)

Steps to Reproduce

1. Configure IBus, with Ergo-L as keyboard layout.
2. Try to hit the “typo” key (o in qwerty) and another letter (e.g. s or q, which are in the same position as qwerty, or e which is d in qwerty)

Expected Behavior

Normally, in Ergo-L only:

• typo then s = é
• typo then q = â
• typo then typo = dead_diaeresis .
• typo then typo then e = ë.

With GTK applications, pressing and releasing typo becomes dead_diaeresis with preedit (¨):

• typo then s = ¨s, typo maintained and s = ¨é
• typo then q = ¨q, typo maintained and q =¨â
• typo then typo then e then e then e etc. = ë¨e¨e¨ etc.

With Qt applications, pressing and releasing typo becomes dead_diaeresis like with GTK, but without the preedit for (¨):

• typo then s = s (typo maintained works correctly)
• typo then q = q (typo maintained works correctly)
• typo then e = ë (typo maintained works correctly)
• typo then typo then e = ë (in this case, the preedit appears)